This invention relates to a circuit arrangement for A.C. operation of at least one gas discharge lamp provided with a full-wave rectifier. The full wave rectifier is to be connected to an A.C. voltage source and has output terminals connected to a combinatorial circuit part in the form of a direct voltage converter, to which is connected a bridge circuit comprising at least two thyristors and in whose transverse branch the lamp is included. The term "thyristor" is to be understood herein to mean an electrical circuit element which is switched to the conducting state by a signal at its gate electrode, but which is switched to the non-conducting state only after the current has fallen below its hold current value. This also applies, for example, to the so-called triacs. The current fall is conventionally achieved by the zero passage of the voltage applied.
Such a circuit arrangement is known from DE OS No. 3136919. In this circuit arrangement, a combinatorial circuit part operating as a down converter produces a pulsatory direct voltage on which a high frequency may be superimposed. This pulsatory voltage is converted by a thyristor bridge, driven by the A.C. voltage source, into an A.C. voltage which varies with the frequency of the voltage source and on which may again be superimposed a given high-frequency modulation. In the known circuit arrangement, the down converter is fed by the A.C. voltage of the source rectified by means of a full-wave rectifier without the rectifier being followed by a smoothing capacitor.
However, such a circuit arrangement can only be used in given lamp types because, in the absence of a smoothing capacitor, reignition difficulties often occur during current zero passages. More particularly in high-pressure gas discharge lamps, it has been found in practice that generally smoothing capacitors of about 0.5 .mu.F to 10 .mu.F are required to maintain a residual ionization of the lamp in the zero passage of the current. The voltage at a smoothing capacitor, and hence at the thyristor bridge, does not fall to zero, however, upon the voltage zero passage of the source. Therefore, when using a smoothing capacitor, shortcircuits may occur in the thyristor bridge circuit, which may lead to extinction of the lamp and even to destruction of the circuit arrangement itself.
U.S. Pat. No. 4,042,856 discloses a similar circuit arrangement having an additional smoothing capacitor, in which the bridge circuit comprises four controlled transistors. In this case, the switching of the transistors is synchronized with the switching of the switching transistor in the combinatorial circuit part. Since transistors are switched into the conducting and non-conducting state by control at their gate electrodes, in this circuit arrangement practically no shortcircuit problems can occur in the transistor bridge.